An industrial, scientific and medical (ISM) band at 2.4 GHz is used in wireless LAN (WLAN) communications according to the IEEE 802.11 standard, such as direct sequence spread spectrum (DSSS) wireless communications, etc. Also proposed is a short-distance wireless standard Bluetooth™, an extremely convenient technology capable of making connection between associated electronic equipments without using a cable, in an ISM band of 2.4 GHz, the same as in the wireless LAN (WLAN).
A major wireless LAN standard utilizing an ISM band of 2.4 GHz has IEEE 802.11b and IEEE 802.11g. The IEEE 802.11b is a DSSS system, supporting high-speed communications at 5.5 Mbps and 11 Mbps. The IEEE 802.11g uses an orthogonal frequency division multiplex (OFDM) modulation system, supporting high-speed data communications at 54 Mbps at maximum.
Bluetooth divides the ISM frequency band of 2.4 GHz to pluralities of wireless channels, each of which is then divided by a unit time ( 1/1600 seconds) to time slots. The wireless channels used are switched by the time slots to provide a frequency-hopping system with excellent noise resistance.
Wireless LAN utilized by small groups in a short distance range within about 50-100 m has such a high data transmission speed as several Mbps to several tens of Mbps, consuming power of about 100 mW. In Bluetooth utilized in a relatively narrow area such as the same compound or building, etc., however, the reach of electromagnetic waves is as short as about 10 m, and its transmission speed is 2 Mbps at most, with as small power consumption as about 10 mW. Because wireless LAN and Bluetooth are different in transmission speed and reach, etc., both of them may be used in one communications apparatus, so that a more advantageous one is used depending on applications.
Explanation will be made below, for instance, with wireless LAN (IEEE 802.11b, IEEE 802.11g) as a first communications system, and Bluetooth as a second communications system.
JP2001-24579 A discloses a circuit usable for both circuits of wireless LAN and Bluetooth, which comprises a first switch circuit for switching the connection of a first antenna port to a transmitting circuit for a first communications system and a second switch circuit, the second switch circuit switching the connection of the receiving circuit of the first communications system to the first switch circuit and a third switch circuit, the third switch circuit switching the connection of the second antenna port to a transmitting/receiving circuit for the second communications system and the second switch circuit; a first filter disposed between the first switch circuit and a transmitting circuit for the first communications system; and a second filter disposed between the second switch circuit and the receiving circuit of the first communications system, as shown in FIG. 35.
With the circuit described in JP2001-24579 A, however, the miniaturization of communications apparatuses is difficult, (a) because it needs two filters, a first filter between the first switch circuit and the transmitting circuit for the first communications system, and a second filter between the second switch circuit and the receiving circuit of the first communications system, and (b) because the receiving circuit of the first communications system is constituted such that it is connected to the first and second antenna ports, resulting in a complicated circuit.
JP2002-208874 A discloses composite wireless device using substantially the same frequency band, which comprises a first splitter for inputting first and second outputs, a power amplifier for inputting the output of the first splitter, an antenna switch for inputting the output of the power amplifier, a low-noise amplifier on the receiving side of the antenna switch, and a second splitter for inputting the output of the low-noise amplifier, the transmitting circuit having such a structure that an output one of different two transmitting signals is selected by the first splitter and then amplified by the power amplifier, and the receiving circuit having such a structure that received waves are distributed to two signals by the second splitter disposed on the output side of the low-noise amplifier, as shown in FIG. 36. In this composite wireless device, the power amplifier and the low-noise amplifier are used for both signals. Specifically, the power amplifier is used for both the transmitting waves of wireless LAN and the transmitting waves of Bluetooth. However, because recent Bluetooth having low transmitting output power does not need a power amplifier, such common use has become unnecessary.